Examples of capacitors include a solid electrolytic capacitor having low ESR (equivalent series resistance) used in a periphery of a CPU (central processing unit) of a personal computer, an aluminum electrolytic capacitor used for smoothing a power supply circuit, and the like. These capacitors have been strongly demanded to have a smaller size and larger capacity.
For example, a solid electrolytic capacitor includes electrode foil (anode foil) having an oxide film formed on the surface thereof, a solid electrolyte layer made of a conductive polymer and formed on the oxide film, and a cathode current collector formed on the solid electrolyte layer. The solid electrolyte layer and the cathode current collector form a cathode layer.
Electrode foil can be obtained by etching metal foil such as aluminum foil. Thus, the surface area of the electrode foil can be increased and the capacitance can be enhanced.
However, there is a technical limit to making finer by etching. Therefore, recently, use of aluminum alloy foil, which contains different metal in aluminum, as metal foil has been studied. Furthermore, it is also studied to enhance the dielectric constant of an oxide film and to increase capacity by changing a composition of metal foil itself by using aluminum alloy foil.
Examples of the above-mentioned aluminum alloy foil include aluminum-zirconium alloy foil containing zirconium in aluminum. It is thought that, because the dielectric constant of an oxide of zirconium is high, use of the aluminum-zirconium alloy foil enhances the dielectric constant of an oxide film and thus contributes to improve the capacitance.
In this way, conventionally, in the electrode foil made of the aluminum-zirconium alloy, the atomic ratio of zirconium is increased so as to enhance the dielectric constant of the oxide film and thereby to enhance the capacitance.
However, in conventional electrode foil made of the aluminum-zirconium alloy, as the atomic ratio of zirconium is increased, the capacitance is rather reduced even if etching processing is carried out.
This is because zirconium cannot be sufficiently dispersed in aluminum, and zirconium reacts with aluminum to precipitate as an intermetallic compound in electrode foil. When the electrode foil is etched, the intermetallic compound is not etched and remains in a state in which it is exposed on the surface of the electrode foil. As a result, the surface area of the electrode foil cannot be sufficiently enlarged, resulting in lowering the capacitance (see, for example, Japanese Patent Application Unexamined Publication No. S60-66806).